The present invention, relates to a mobile communication system which supports multiple-base station coordination service and a remote radio head clustering method thereof, and has high flexibility and saves radio resources.
Along with an increase in mobile users and a high request for a data rate, a conventional cellular network fails to satisfy needs of a number of users more and more. 3rd-Generation Partnership Project (The 3GPP) being an international standardization organization develops a system architecture and specifications of second generation and third generation mobile communication networks, and the specifications are currently already applied to a network in which an air interface is arranged. At present, the 3GPP makes start on an establishment of Long Term Evolution Version (LTE) and Long Term Evolution-Advanced Version (LTE-A) in preparation for a fourth generation mobile communication network. As its aim, the 3GPP increases a spectrum utility factor (throughput/bandwidth) of a system and particularly a spectrum utility factor of a cell edge.
However, since using the same frequency in adjacent cells, an LTE/LTE-A system causes strong interference between cells. When radio modules of a base station are arranged in a separate place by using an optical fiber, they serve as remote radio heads. They are distributed and arranged in the base station determined by a network scheme, and cover a cover range of an original integral base station. Each remote radio head uses small transmission power and as a result, obviously reduces interference between cells. This is a concept of a first distribution antenna system.
Further, a Coordinated Multi-point Transmission/Reception (CoMP) is used as one of methods for effectively reducing interference between cells. In the second generation and third generation mobile communication networks, one cell provides service for one user. Hereinafter, it is abbreviated to a “single point transmission”. The single point transmission is to perform transmission and reception from a pair of antennas very close to each other. A normal distance between the above antennas is several wavelengths, and they receive fading for the same long time. A pair of antennas is referred to as one “point”. In the following embodiment, for example, any of “one base station having a pair of omnidirectional antennas”, “one sector of a base station divided into sectors”, one “home base station”, one “relay station”, and one “remote radio head” in a distribution antenna system can be considered as the “single point transmission”. A cover range of one “point” is known generically as a “cell”. A “multi-point transmission” is to perform transmission and reception from a plurality of antenna groups not close to each other. A normal distance between these antenna groups is several wavelengths, and they receive fading for the different long time. In the following embodiment, for example, any of a CoMP between base stations, a CoMP between sectors (including multiple sectors of the same base station and multiple sectors of different base stations), a CoMP between multiple remote radio heads, and a CoMP between a base station and a relay station can be considered as a “multi-point coordinated transmission/reception”. In the CoMP, beam forming can be performed based on a distance between user equipment and each point, and interference between cells can be removed or used. In the patent, for studying the distribution antenna system, a so-called point serves as a remote radio head. When using a pair of omnidirectional antennas, each remote radio head has a separate cell ID.
The distribution antenna system of a conventional technology will be described below with reference to FIG. 8.
FIG. 8 illustrates a typical service scene of the distribution antenna system based on a conventional LTE/LTE-A base station. The base station 102 is connected to a gateway 101 and accesses other networks. The base station 102 includes a baseband unit 1021, and remote radio heads 1041 to 1043, and 1051 to 1053. The baseband unit 1021 performs, as a main function, baseband processing of a signal, including coding/decoding, and modulation/demodulation. The remote radio head 1041-1043, 1051-1053 performs, as a main function, an A/D conversion of signals, frequency conversion, filtering, amplification, and radio transmission. The remote radio heads 1041 to 1043 and the remote radio heads 1051 to 1053 are connected via an optical fiber 1023 and the baseband units 1021. In this distribution antenna system, the COMP is used. At one time, a service received by user equipment 103 is a coordination transmission of two remote radio heads 1041 and 1042 belonging to the same base station. On the other hand, a service received by user equipment 104 is a coordination transmission of three remote radio heads 1043, 1051, and 1052 belonging to different-base stations.
However, since receiving a limitation of a peculiar constitution of a cellular cell, the above-described distribution antenna system has various insoluble problems. For example, any of respective base stations of the system ought to make a monopoly of one base station address. It means that independent machine room, air conditioner, and electric-supply equipment ought to be put in place, and the above-described constitution is accompanied by enormous cost in the construction and maintenance of the base station. In the LTE system, a concept of the CoMP is further proposed. However, radio resources are allocated only to a limited coordinated cell, and therefore, large-sized resources fails to be optimized. For preventing the above-described problem, a new system architecture needs to be introduced into a future high-speed radio communications system. A distribution antenna system based on a centralized baseband pool is a new radio access system adapted to this request.
The distribution antenna system based on the centralized baseband pool of the conventional technology will be described below with reference to FIG. 9.
FIG. 9 is a system frame diagram of the distribution antenna system based on the centralized baseband pool. This system includes two important portions, and one portion is a distribution antenna network 204 including a plurality of remote radio heads 211 to 226 which transmit signals to user equipment, and the plurality of remote radio heads are flexibly arranged by using an optical fiber remote technology. The other portion is the centralized baseband pool 201, baseband portions of all the base station in the system are concentrated into one place to form one baseband pool. A resource allocation having high efficiency is performed by using a high-speed signal processing technology. The centralized baseband pool can largely reduce the number of necessary machine rooms, share matching of radio resources with the network, and is advantageous to an application of the CoMP technology. These two portions are connected via an improved optical transmission network 203, and a switcher 202 is controlled in real time by ports of the remote radio heads which perform service.
By using the CoMP technology, any of the above-described distribution antenna systems also can reduce interference between cells and improve a spectrum utilization factor of a cell. Obviously, it is unrealistic that all the remote radio heads of the entire network cooperate. After all, matters such as coordination is performed by using how many points, and how coordination is performed are a clustering problem on the remote radio head about which we just dispute. The clustering is to group the remote radio heads according to a certain rule. When the multi-point coordinated transmission/reception service is performed to users in the remote radio head of each group, throughput of the user can be sufficiently improved.
Static clustering and dynamic clustering of the remote radio head in the distribution antenna system of the conventional technology will be each described below with reference, to FIGS. 10, 11A and 11B.
FIG. 10 is a schematic diagram illustrating static clustering of the remote radio head of the distribution antenna system. In FIG. 10, a cluster size is 4. The “static clustering” is to already group the remote radio heads into clusters according to a fixed model in a planning stage of the system. After the network is planned, the clusters fail to be changed. As illustrated in FIG. 10, for example, 16 remote radio heads of the entire network are grouped into four clusters. Any size (the number of the remote radio heads in the cluster) of the clusters 1, 2, 3, and 4 is 4. User equipment which moves to each cluster receives only service from four remote radio heads of the cluster. The above-described clustering method is relatively simple, has low flexibility, and the throughput of the system is limited. For example, the user equipment D1 of FIG. 10 is located at the center of the cluster 1 and the cluster 1 provides preferable service for the user equipment D1. On the other hand, the user equipment D2 is located at the edge between the clusters 1 and 2, when the cluster 2 provides service for the user equipment 2, large interference occurs with respect to the cluster 1.
FIGS. 11A and 11B are schematic diagrams illustrating dynamic clustering of the remote radio head of the distribution antenna system. In FIGS. 11A and 11B, a cluster size is 2. The “dynamic clustering” is to dynamically change a configuration of the cluster between one period of time to another period of time. For example, FIG. 11A illustrates a clustering result of the time N, and FIG. 11B illustrates a clustering result of the time N+1. In the above-described method, a comprehensive judgment is normally performed based on channel state information of real time between mobile facilities and each remote radio head. Further, the method has a complicated algorithm, high flexibility, and high throughput of the system.
A multi-point transmission problem of the distribution antenna system is sometimes mentioned also in the conventional technology.
In WO 2010/077192 A1, a method for measuring a subcell in the distribution antenna system is disclosed. Practically speaking, the above-described distribution system is conventionally a wireless remote system based on the base station. In the patent, a “concept of the subcell” corresponds to an area covered by one Remote Radio Head (RRH) of the patent. A pair of subcells provides service for mobile user equipment in a coordinated manner. However, in this patent, it is not mentioned that by using what method, multiple “subcells” are combined together and service is provided for mobile user equipment in a coordinated manner. It is simply mentioned that service is provided for the mobile user equipment in a coordinated manner by using two or more subcells.
In CN 101777941A, a downlink transmission method of the multi-point transmission system is proposed; however, a method for clustering a coordinated cell is not specifically mentioned.
In CN 101185270 A, a dynamic distribution, antenna selection system of a wireless communication base station and an implementation method thereof are disclosed. Practically speaking, the above-described distribution antenna system is conventionally a wireless remote system based on the base station. The implementation method of this invention is to select N pieces of antennas from among K pieces of antenna units for transmission. In the patent, it is simply introduced that a method for selecting an antenna is to perform a dynamic selection based on obstacles of environment, a type of work, and a position of a user, and a specific algorithm is not described.
In CN 101631379, power allocation and antenna selection method in a distribution antenna system are proposed. A base station takes the statistics of received antenna signal strength of each user equipment for averaging in a predetermined time, performs power allocation only to an antenna having the averaged antenna signal strength larger than a threshold, and selects an antenna having preferable radio channels, thus implementing a communication with mobile user equipment. However, since selecting an antenna based on an uplink signal strength, the above-described, method is not applied to an FDD system which is not related to an uplink and a downlink.